The invention is useful, for example, in the floater air dryer section of a machine for processing wood fiber pulp. In a typical pulp machine, the pulp web passes through a series of mechanical presses and/or dryers and then through in a floater air dryer section. The web is floated between a series of decks of nozzle boxes out of which hot air is ejected for the dual purpose of supporting and drying the web. In the normal arrangement of this section of a pulp machine, a series of horizontal nozzle or blow boxes form a deck to which hot air is supplied through ends of the boxes near one side of the web path. Air is ejected vertically from the individual nozzle boxes from opposite directions toward the web path to both float and dry the web as it passes through a series of vertically arranged horizontal passes of the dryer. The stability of web-floating, rate of heat transfer, and evaporation rate depend upon the manner of impingement of the air on the web.
In at least one type of operation, it is recognized as desirable to direct the air from the nozzle boxes in a direction at right angles with the web path. Current designs either do not achieve the perpendicular air flow, or they make use of turning vanes, straighteners, or perforated sheets placed beneath the exit air apertures in the nozzle boxes to obtain the desired mode of impingement. Any such constructions require the use of material for the "straighteners" and the expense of fan horse power to overcome the pressure loss against the apertures resulting from "straightening" the air.
Attempts were made through testing on full scale dryer equipment having nozzle boxes which did not include auxiliary structure, such as straighteners, turning vanes, perforated sheets etc. Floating motion and floating instability of the web was encountered which was eventually determined to be caused by the non-vertical or non-perpendicular direction relative to the web path of the exit path of the air from the apertures in the nozzle boxes. Many variations of apertures were tested until a satisfactory design was found.
Objects of such experimentation were to find a nozzle box design capable of discharging jets of air in the desired direction which was simple in design and economical to construct but could be operated efficiently from an energy standpoint without undesirable behavior of the web. A nozzle box design was discovered which is arranged as herein disclosed to provide an aperture and air-deflecting tab design which has proved to be highly efficient and to cause virtually no pressure loss at dryer design operating levels. A substantial further benefit of this design is its ease of manufacture.